The present invention relates to the preparation of tetraalkyl ammonium tetrafluoroborates and to the intermediates thereof. More particularly, there is provided a process for preparing tetraalkyl ammonium tetrafluoroborates from tetraalkyl ammonium halide salts.
Tetraalkyl ammonium tetrafluoroborates are useful in non-aqueous capacitors.
Tetraalkyl ammonium halides preparation has been described in U.S. Pat. No. 3,965,178 to Morris et al and in Japanese Patent No. 2000212132 to Shimada et al., and by Hossain et al. in J. of Bangladesh Acad. Of Sciences. Vol. 22, No. 1, 39-44(1998) in an article entitled xe2x80x9cKinetics of The Menschutkin Reaction of Pyridine With Benzyl Bromide In Different Solventsxe2x80x9d.
The preparation of tetraalkyl ammonium halide salts by the Menschutkin reaction is as follows:
RX+Rxe2x80x23Nxe2x86x92RRxe2x80x23NX
where X=I, Br, Cl
This reaction is usually run in various solvents both protic and aprotic. The reaction runs faster in aprotic solvents such as benzonitrile, acetonitrile, tolene, nitrobenzene, propionitrile, acetone, etc. The rate of reaction is RI greater than RBr greater than RCl for the alkyl halides. The more bulky or substituted the reactant, the slower the reaction runs. Methyl bromide reacts much faster than ethyl bromide, which is faster than 2-bromopropane. The same is true of the alky chlorides, which are slower to start with. Trimethyl amine reacts faster than triethyl amine with the alkyl halide due to steric effects. The same steric effects are true with the alkyl halides. One of the slowest reacting situations is the reaction of an alkyl chloride with a trialkyl amine such as triethyl amine. For example:
Ethyl chloride+Et3Nxe2x86x92Et4NCl 
The present invention provides a method for the preparation of tetraalkyl ammonium halides by the use of a catalytic amount of acetonitrile under pressure and elevated temperatures, which can be used in a further process for preparing tetraalkyl ammonium tetrafluoroborates.
More particularly, tetraalkyl ammonium halides can be prepared under an increased reaction rate and high yield wherein in the same reaction vessel can be prepared tetraalkyl ammonium tetrafluoroborate in high yields and purity.
According to the process, the acetonitrile can be removed after the initial preparation of the tetraalkyl ammonium halides and recycled into subsequent reactors.
Alternatively, the tetraalkyl ammonium halides can be recovered for other uses and for subsequent reactions to prepare tetraalkyl ammonium tetrafluoroborates.
It is therefore an object of the invention to provide a novel process for preparing tetraalkyl ammonium halides in high yields.
It is another object to prepare tetraalkyl ammonium tetrafluoroborates.
It is yet another object to provide a continuous process for preparing tetraalkyl ammonium tetrafluoroborates utilizing tetraalkyl ammonium halides as intermediates.
These objects and advantages of the invention will be more clearly understood from a reading of the preferred embodiments and examples, which follow.
According to one embodiment of the invention, there is prepared tetraalkyl ammonium halides of the formula:
RRxe2x80x23NX
wherein
R is an alkyl group of 1 to 18 carbon atoms,
Rxe2x80x2 is an alkyl group of 1 to 8 carbon atoms, and
X is chloro, bromo or iodo.
Preferably, R and Rxe2x80x2 are methyl, ethyl or propyl groups.
The tetraalkyl ammonium halides are prepared by reacting a trialkyl amine having 3 to 8 carbon atoms with at least a 20% stochiometric excess of an alkyl halide having up to 18 carbon atoms, preferably 1 to 8 carbon atoms in the presence of a catalytic amount of acetonitrile, preferably about 5 to 50% of the reaction mass at a temperature of about 70 to 180xc2x0 C., preferably, about 100 to 150xc2x0 C. at a pressure of about 80 to 200 psi, preferably, about 160 to 180 psi.
It has been found that the reaction rate and the subsequent yield was increased by the following means:
1. Use of at least 20% to 100% excess of the stoichiometric amount of alkyl halide.
2. Use of a catalytic amount of at least 5 to 50% by weight of the reaction mass of acetonitrile, preferably 20 to 40% by weight.
The tetraalkyl ammonium halide can be recovered after the removal of the volatile by conventional means and then dissolving the remaining solid product in water or methanol.
According to another embodiment of the invention, after removal of the volatiles, tetraalkyl ammonium tetrafluoroborate can be prepared by the addition of aqueous fluoroboric acid into the reaction mixture. The fluoroboric acid can be prepared in situ with hydrofluoric acid and boric acid.
The process has the advantage that there may be a continuous process whereby the acetonitrile excess is used in a subsequent reactor.
The preferred alkyl halides are methyl chloride, ethyl chloride and propyl chloride.
The preferred trialkyl amines are trimethyl amine, triethyl amine, alkyl dimethylamine, methyldiethyl amine and tripropyl amine.
It is further understood that the preparation of tetraalkyl ammonium tetrafluoroborate can be prepared using tetraalkyl ammonium halides prepared by prior art means.
The following examples are provided to show specific embodiments and are given for illustration purposes and not by way of limitation: